Landscaping materials, such as wood chips, wood mulch, gravel, sand, rubber chips, and rubber mulch, are currently available in their normal natural colors, and in some cases, specifically wood chips and wood mulch, are available colored, for example, in black, brown, and red. Rubber chips are available painted for use as playground material. Gravel and sand used in landscaping applications are not commonly available in non-natural colors.
The conversion of natural wood or other plant fiber-based substrates to colored substrates is currently a rather sloppy, inefficient, expensive, and, in some cases, environmentally unfriendly process. In the case of converting natural mulch to colored mulch, or natural chips to colored chips, the process circumstances are highly inefficient. In order to color mulch or chips, a producer of such colored product typically purchases a pigment suspension of an appropriate color, which, if it is carbon to produce a black product, typically ranges from about ten pounds per gallon to about eleven pounds per gallon, and about twenty-five to fifty weight percent carbon pigment. Typically, the carbon suspension is distributed in 55-gallon drums weighing approximately 500 net pounds and containing about 150 to 250 pounds of carbon. Currently, in order to apply this concentrated pigment dispersion to the substrate to be colored, the producer dilutes it with a substantial volume of water, a 60:1 dilution in some cases, bringing the final volume from 55 gallons to as much as 3300 gallons or more. The mulch is essentially then slurried in this highly diluted pigment dispersion. This results in a water saturated product which must be allowed to dry for extended periods of time before it can be used, and still may contain excessive water, adding significantly to the shipping weight and thus the shipping costs.
Some currently available mulch colorant devices can process mulch at the rate of 80-200 cubic yards per hour, consuming about 25-70 gallons of water per minute, or about 1500-4200 gallons of water per hour. This volumetric flow rate converts to about 15 to 20 gallons of water per cubic yard of mulch treated. At a mid-range black color level using a carbon black pigment dispersion, and 200 cubic yards per hour, 3.33 cubic yards per minute, the carbon dispersion feed rate is about 0.5 to 1.5 gpm, corresponding to about 5 to 15 pounds per minute of carbon dispersion or about 1.3 to 5.3 pounds of carbon pigment per minute, where the dispersion is approximately 25 to 35 percent carbon by weight. This corresponds to about 0.4 to 1.6 pounds of carbon pigment per cubic yard of mulch.
Assuming a pigment dispersion having 30 weight percent carbon pigment, in one hour, 200 cubic yards of mulch will be colored, using 3600 gallons of water and 666 pounds of carbon dispersion (or 200 pounds of carbon pigment). Since the wet colored mulch weighs about 700 pounds per cubic yard, the total mass of mulch, wet, is approximately 140,000 pounds, including the added total weight of water of about 30,000 pounds, along with 666 pounds of carbon dispersion, or 30,700 pounds total, added weight. Depending on the substance or substances from which the mulch is made, the untreated mulch may contain anywhere from 10 weight percent to 75 weight percent moisture, from very dry pallet wood to mulch exposed to excessive rainfall. Where the moisture content of the mulch material is essentially about 60+ weight percent moisture, there is little capacity to absorb significant additional water, meaning that the coloring water solution will be mostly wasted.
If the mulch is able to absorb 10 percent moisture weight from the colored water solution, as an example, approximately 13,000 pounds of water would be absorbed and 19,000 pounds, 2278 gallons (60 percent), would be lost as overflow, runoff, or post-treatment bleeding. This is not only an inefficient use of materials, but, depending on the colorant being employed, may also be an environmental hazard.
In the case of coloring rubber chips, a pigmented polymeric composition, essentially paint, is generally used to coat the rubber chips by dip coating, i.e., submersion of the chips in the liquid composition. In this process, the resultant colored rubber chips are covered in wet paint, and therefore the unused paint needs to be separated and the coated rubber chips dried. These coating compositions are water-based so the drying process is either slow at ambient conditions or energy consumptive at elevated (dryer) conditions. The residual unused coating composition is not recoverable, and since this composition is expensive, the loss of material adds to the overall cost of production.
A process capable of more efficient water and pigment consumption, and of reduced moisture content of the colored product would benefit the overall economics and environmental consequences of these coloring processes.